Method for the treatment of polyazole films

ABSTRACT

The present invention relates to a process for the treatment of polyazole films, in which a film is passed at least twice through a trough filled with a liquid, with the film being unrolled from a spool and rolled up on a further spool and the direction of travel of the film is changed during the treatment by altering the direction of rotation of the spools.

The present invention relates to a process for the treatment ofpolyazole films with liquids.

Owing to their excellent chemical, thermal and mechanical properties,acid-doped polymer membranes can be used in a variety of applicationsand are particularly suitable as polymer electrolyte membrane (PEM) inPEM fuel cells.

The basic polyazole membranes are doped with concentrated phosphoricacid or sulfuric acid and act as proton conductors and separators inpolymer electrolyte membrane fuel cells (PEM fuel cells).

Due to the excellent properties of the polyazole polymer, such polymerelectrolyte membranes can, when processed to produce membrane-electrodeunits (MEUs), be used in fuel cells at long-term operating temperaturesabove 100° C., in particular above 120° C. This high long-term operatingtemperature allows the activity of the catalysts based on noble metalswhich are present in the membrane-electrode unit (MEU) to be increased.Particularly when using reformats of hydrocarbons, significant amountsof carbon monoxide are present in the reformer gas and these usuallyhave to be removed by means of a costly gas work-up or gas purification.The ability to increase the operating temperature enables significantlyhigher concentrations of CO impurities to be tolerated over the longterm.

The use of polymer electrolyte membranes based on polyazole polymersallows, firstly, the costly gas work-up or gas purification to beomitted in some cases and, secondly, the catalyst loading in themembrane-electrode unit to be reduced. Both are dispensableprerequisites for large-scale use of PEM fuel cells, since otherwise thecosts of a PEM fuel cell system are too high.

Doping with acids alters the mechanical properties of the filmappreciably. Thus, for example, the modulus of elasticity decreases to10% of the initial value, so that a polyazole film has only a relativelylow mechanical stability after doping. In addition, the area of the filmincreases by up to 50% as a result of doping.

Owing to these problematical properties, these films are doped with acidin a purely manual process by laying the films in a water or acid bathand subsequently changing the liquid bath a number of times.

A problem associated with this method is, inter alia, the highconsumption of liquid. Furthermore, processes according to the prior artare not very flexible and are very labor intensive. It should be notedthat polyazole films initially display a very low flexibility but thisincreases greatly on treatment with acids while the mechanical stabilityis reduced.

Furthermore, such films which are generally cast from organic solventscontain high proportions of solvent residues which can be hazardous tohealth. In the past, these solvents, for example dimethylacetamide(DMAc) have been removed by drying.

It is therefore an object of the present invention to provide a processwhich solves the abovementioned problems. The process should allow asimple, safe and reliable treatment of polyazole films.

It is a further object of the present invention to provide a processwhich is very flexible and can be adapted to the considerable changes inthe mechanical behavior of the film occurring during the treatmentwithout thereby increasing the labor requirements.

In addition, the process should have a particularly low consumption ofliquid. The process should also be inexpensive.

It is therefore also an object of the present invention to provide aprocess which makes removal of hazardous solvents possible. Inparticular, the process should allow both removal of solvents and dopingof polyazole films without different plants being necessary.

Furthermore, the process should improve the mechanical stability ofpolyazole films. These properties include, in particular, the modulus ofelasticity, the tear strength and the fracture toughness of the film.

These objects are achieved by a process for the treatment of polyazolefilms, in which a film is passed at least twice through a trough filledwith a liquid, with the film being unrolled from a spool and rolled upon a further spool and the direction of travel of the film is changedduring the treatment by altering the direction of rotation of thespools.

In the field of textiles, this procedure is known, for example, indyeing. However, in contrast to fabrics, a film is not able to take uplarge amounts of liquid within a short time. Furthermore, textilesessentially retain their mechanical properties and their dimensionsalter only slightly during the treatment.

Accordingly, the solution provided by the invention is particularlysurprising because the process adapts to the changing properties of thefilm. In addition, the film comes into contact with the liquid presentin the trough for only a very short time without this having an adverseeffect on the treatment. Surprisingly, it therefore has to be assumedthat the treatment of the film also takes place in the roiled-up stateby means of liquid which is rolled up together with the film.

According to the invention, films comprising polyazoles are treated.Before treatment with acid, these films generally comprise at least 50%by weight, preferably 70% by weight, of polyazoles, without thisconstituting a restriction. Such polymers are known per se.

Polyazoles comprise recurring azole units of the general formula (I)and/or (II) and/or (III) and/or (IV) and/or (V) and/or (VI) and/or (VII)and/or (VIII) and/or (IX) and/or (X) and/or (XI) and/or (XII) and/or(XIII) and/or (XIV) and/or (XV) and/or (XVI) and/or (XVII) and/or(XVIII) and/or (XIX) and/or (XX) and/or (XXI) and/or (XXII)

where

-   the radicals Ar are identical or different and are each a    tetravalent aromatic or heteroaromatic group which can be monocyclic    or polycyclic,-   the radicals Ar¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar² are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which can be    monocyclic or polycyclic,-   the radicals Ar³ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁴ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁵ are identical or different and are each a    tetravalent aromatic or heteroaromatic group which can be monocyclic    or polycyclic,-   the radicals Ar⁶ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁷ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁸ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁹ are identical or different and are each a divalent    or trivalent or tetravalent aromatic or heteroaromatic group which    can be monocyclic or polycyclic,-   the radicals Ar¹⁰ are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which can be    monocyclic or polycyclic,-   the radicals Ar¹¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals X are identical or different and are each oxygen,    sulfur or an amino group bearing a hydrogen atom, a group having    1-20 carbon atoms, preferably a branched or unbranched alkyl or    alkoxy group, or an aryl group as further radical,-   the radicals R are identical or different and are each hydrogen, an    alkyl group or an aromatic group and-   n, m are each an integer greater than or equal to 10, preferably    greater than or equal to 100.

Preferred aromatic or heteroaromatic groups are derived from benzene,naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine,tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole, benzotriazole,benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzopyrazine, benzotriazine,indolizine, quinolizine, pyridopyridine, imidazopyrimidine,pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline,phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthrolineand phenanthrene, each of which may also be substituted.

Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have any substitutionpattern; in the case of phenylene, for example, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸,Ar⁹, Ar¹⁰, Ar¹¹ can be ortho-, meta- or para-phenylene. Particularlypreferred groups are derived from benzene and biphenylene, each of whichmay also be substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, e.g. methyl, ethyl, n-propyl or isopropyl and t-butylgroups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as fluorine, amino groups,hydroxyl groups or short-chain alkyl groups such as methyl or ethyl.

Preference is given to polyazoles having recurring units of the formula(I) in which the radicals X within a recurring unit are identical.

The polyazoles can in principle also have differing recurring unitswhich, for example, differ in their radical X. However, there arepreferably only identical radicals X in a recurring unit.

Further preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

In a further embodiment of the present invention, the polymer comprisingrecurring azole units is a copolymer or a blend comprising at least twounits of the formulae (I) to (XXII) which differ from one another. Thepolymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer comprising recurring azole units is a polyazole containing onlyunits of the formula (I) and/or (II).

The number of recurring azole units in the polymer is preferably greaterthan or equal to 10. Particularly preferred polymers comprise at least100 recurring azole units.

For the purposes of the present invention, preference is given topolymers comprising recurring benzimidazole units. Some examples of theextremely advantageous polymers comprising recurring benzimidazole unitsare represented by the following formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

Preferred polyazoles, but in particular the polybenzimidazoles, have ahigh molecular weight. Measured as intrinsic viscosity, this is at least1.0 dl/g, preferably at least 1.2 dl/g.

The preparation of such polyazoles is known: one or more aromatictetraamino compounds are reacted in the melt with one or more aromaticcarboxylic acids or esters thereof which contain at least two acidgroups per carboxylic acid monomer to form a prepolymer. The prepolymerformed solidifies in the reactor and is subsequently broken upmechanically. The pulverulent prepolymer is usually fully polymerized ina solid-state polymerization at temperatures of up to 400° C. Preferredpolybenzimidazoles are commercially available under the trade name®Celazole from Celanese AG.

To produce polymer films, the polyazole is, in a further step, dissolvedin polar, aprotic solvents such as dimethylacetamide (DMAc) and a filmis produced by classic methods.

To remove solvent residues, the film obtained in this way can be treatedwith a washing liquid by means of the process of the invention. Thiswashing liquid is preferably selected from the group consisting ofalcohols, ketones, alkanes (aliphatic and cycloaliphatic), ethers(aliphatic and cycloaliphatic), esters, carboxylic acids, with the abovemembers of the group being able to be halogenated, water, inorganicacids (e.g. H₃PO₄, H₂SO₄) and mixtures thereof.

In particular, C1-C10-alcohols, C2-C5-ketones, C1-C10-alkanes (aliphaticand cycloaliphatic), C2-C6-ethers (aliphatic and cycloaliphatic),C2-C5-esters, C1-C3-carboxylic acids, dichloromethane, water, inorganicacids (e.g. H₃PO₄, H₂SO₄) and mixtures thereof are used. Among theseliquids, water is particularly preferred.

After washing, the film can be dried to remove the washing liquid.Drying is carried out as a function of the partial vapor pressure of thetreatment liquid selected. Drying is usually carried out at atmosphericpressure and temperatures of from 20° C. to 200° C. More gentle dryingcan also be carried out under reduced pressure. In place of drying, themembrane can also be dabbed off and thus freed of excess treatmentliquid. The order is not critical.

The above-described freeing of the polyazole film of solvent residuessurprisingly improves the mechanical properties of the film. Theseproperties include, in particular, the modulus of elasticity, the tearstrength and the fracture toughness of the film.

To give these films the ability to conduct protons, they are doped withan acid. The present process is particularly useful for this purpose,too. In this context, acids include all known Lewis and Brønsted acids,preferably inorganic Lewis and Brønsted acids.

Furthermore, the use of polyacids, in particular isopolyacids andheteropolyacids, and of mixtures of various acids is also possible. Forthe purposes of the present invention, heteropolyacids are inorganicpolyacids which have at least two different central atoms and are formedas partial mixed anhydrides from in each case weak, polybasic oxo acidsof a metal (preferably Cr, Mo, V, W) and a nonmetal (preferably As, I,P, Se, Si, Te). They include, inter alia, 12-molybdophosphoric acid and12-tungstophosphoric acid.

The conductivity of the polymer of the invention can be influenced viathe degree of doping. The conductivity increases with increasingconcentration of dopant until a maximum value has been reached.According to the invention, the degree of doping is reported as mole ofacid per mole of repeating units of the polymer. For the purposes of thepresent invention, a degree of doping of from 3 to 15, in particularfrom 6 to 12, is preferred.

Dopants which are particularly preferred according to the invention aresulfuric acid and phosphoric acid. A very particularly preferred dopantis phosphoric acid (H₃PO₄). Highly concentrated acids are generally usedhere. According to a particular aspect of the present invention, theconcentration of the phosphoric acid is at least 50% by weight, inparticular at least 80% by weight, based on the weight of the dopant.

Furthermore, it is possible to replace some or all of the acid bymonomers containing vinylphosphonic acid and/or vinylsulfonic acidgroups and thus bring about swelling of the film. The swollen film issubsequently polymerized as described in the German patent application10210499.2 or 10209419.5.

The film is passed through a liquid bath at least twice, preferably atleast 10 times and particularly preferably at least 25 times, with thedirection of travel of the film being changed by altering the directionof rotation of the spools.

The speed at which the film is conveyed through the liquid depends onthe type of liquid and on the film. In general, the film is drawnthrough the liquid bath at a speed of from 0.5 to 100 m/min, inparticular from 1.0 to 25 m/min.

The length of the path of the film through the liquid is preferably from0.05 to 10 m, in particular from 0.15 m to 2 m.

In a particular embodiment of the present invention, the treatment timeis in the range from 2 minutes to 10 hours, preferably in the range from15 minutes to 3 hours.

The speed at which the film is conveyed through the bath of liquid canbe controlled in a manner known per se. Suitable methods include, interalia, control of the speed of rotation of the spools via a speedindicator roller or by measuring their rotation rate.

In a particular embodiment of the present invention, particularlyuniform, smooth and crease-free rolling up of the film is achieved bymeans of the drawback force. The film is preferably conveyed through thetrough filled with liquid with a drawback force per unit width of thefilm in the range from 0.5 to 200 N/m, preferably from 1 to 150 N/m andparticularly preferably in the range from 12 to 60 N/m. The width is inthis case the dimension of the film perpendicular to the direction oftravel prior to treatment with liquid. Based on film having a width inthe range from 20 cm to 200 cm, preferred drawback forces are thus inthe range from 0.1 to 400 N, in 2 particular from 0.2 to 300 N andparticularly preferably from 2.4 to 120 N, without this constituting arestriction.

According to a particular aspect of the present invention, liquidadheres to the film after the first treatment, with preference beinggiven to at least 1 g/m², in particular at least 10 g/m², of liquidremaining on the film after the first treatment. This value is based onthe increase in weight resulting from the treatment with liquid.

In the treatment of the film with a washing liquid, for example water,it is preferred that from 1 to 1000 ml/m², in particular from 5 to 250ml/m², particularly preferably from 15 to 150 ml/m² and veryparticularly preferably from 25 to 75 ml/m², adhere to the film, withoutthis constituting a restriction. If the film is being doped with anacid, for example phosphoric acid, it is preferred that from 1 to 1000ml/m², in particular from 10 to 800 ml/m², particularly preferably from50 to 600 ml/m² and very particularly preferably from 100 to 400 ml/m²,adhere to the film.

The amount of liquid which adheres to the film after a treatment in thebath of liquid and penetrates into the film can be controlled via thespeed with which the film is conveyed through the bath of liquid.

Furthermore, the amount of liquid is dependent on the temperature atwhich the treatment is carried out. The temperature at which the presentprocess is carried out is not critical and can therefore vary within awide range. However, the present process is generally carried out in therange from 0 to 150° C., preferably from 10° C. to 100° C., with theranges depending on the physical properties of the liquid.

In a particular embodiment, the liquid present in the trough can, ifnecessary, be renewed or be replaced by another liquid. In this way, itis possible, for example, to replace a contaminated liquid by a freshliquid of the same type. Furthermore, the liquid can be replaced byanother liquid. This measure enables a film to be both washed and dopedwithout another apparatus having to be used. This procedure can becarried out batchwise or continuously, with individual components alsobeing able to be added.

Jiggers as are described, for example, in Dietmar Fries, trainingmaterial, teaching aids “Textilveredelung, Beschichten”,Arbeitgeberkreis Gesamttextil AGK (1992) p. 2.13, are particularlysuitable for carrying out the present process. These apparatuses arecommercially available from, inter alia, the companies Mathis AG andKuester AG.

The present invention is illustrated below for use of a jigger shownschematically in FIG. 1, without this description restricting theinvention.

A polyazole film (1) is unrolled from a spool (2) and rolled up on asecond spool (3) and in the process conveyed, for example, over a roller(4). The film is conveyed through a trough (5) and there passed around aroller (6). In the trough, the film is treated with liquid. After thefilm has left the trough and been passed around a further roller, excessliquid can, if desired, be removed by means of pressure generated bymeans of a further roller (7) before the film is rolled up. Liquidgenerally adheres to the polyazole films, so that this liquid acts onthe film even in the rolled-up state.

All parts of the jigger which come into contact with the liquid can bemade of nonrusting material. This is particularly advantageous in dopingof the film with concentrated acids. The rollers and spools canaccordingly be made of, for example, stainless steel.

The speed of and/or the drawback force on the film can, for example, bedetermined by means of the rollers (4) and/or (6), which are/is thendesigned as speed indicator roller or tensiometer roller. In addition,the apparatus can be provided with an electronic control system whichregulates the speed and the direction of rotation of the rollers. Theapparatus can in this way be designed so that it automatically changesthe direction of travel after all the film (1) has been transported fromone spool (2) to'the second spool (3).

Furthermore, means of controlling the temperature of the jigger, inparticular the trough (5), can be provided, with the heat energyintroduced into the rolled-up film also depending, in particular, on thespeed of rotation of the spools (2) and (3).

Furthermore, the jigger can have a cover (8) which encloses the troughand the spools and isolates them from the environment. Volatilization ofthe liquid can be prevented in this way. Furthermore, hygroscopicliquids, for example concentrated phosphoric acid, can be protected frommoisture, in which case the jigger can be purged with dry air or withnitrogen.

The invention is illustrated below by means of an example and acomparative example, without the invention being restricted to theseexamples.

EXAMPLE 1

A model WJ 650 jigger from Mathis AG was supplied with about 20 m of PBIfilm which had a width of 42 cm and contained about 25% by weight ofDMAc. The trough of the jigger was filled with 3 l of water. The waterof the jigger was heated to 80° C. The film was treated thirty timeswith the water. The jigger was operated at 3.0 m/min at a drawback forceof 10 N, with the film being immersed for about 5 s each time (length ofthe path through the liquid: about 25 cm).

The residual solvent content of the film was determined by Karl Fischer(KF) titration. Using a Mettler-Toledo apparatus, the water content ofthe film is determined directly by KF titration, as follows. Thespecimen located in a closed sample bottle is heated to 250° C. anddried at this temperature. The gas liberated in this way is passeddirectly into a closed titration vessel and analyzed using Karl Fischer[KF] reagent. In addition to the determination of the water content, theresidual solvent content is determined by gravimetric determination ofthe weight before and after drying. The DMAc content after the treatmentwas 0.0%.

COMPARATIVE EXAMPLE 1

The PBI film used in Example 1 was dried in a conventional manner at 40°C.

The DMAc content measured by the KF method after the treatment was 2%.

1. A process for the treatment of polyazole films, characterized in thata film is passed at least twice through a trough filled with a liquid,with the film being unrolled from a spool and rolled up on a furtherspool and the direction of travel of the film is changed during thetreatment by altering the direction of rotation of the spools.
 2. Theprocess as claimed in claim 1, characterized in that the liquid in thetrough is changed after the first treatment.
 3. The process as claimedin claim 1, characterized in that liquid adheres to the film after thefirst treatment.
 4. The process as claimed in claim 3, characterized inthat at least 1 g/m² of liquid adheres to the film.
 5. The process asclaimed in claim 1, characterized in that the liquid comprises water. 6.The process as claimed in claim 1, characterized in that the liquidcomprises acid, monomers containing vinylsulfonic acid and/or monomerscontaining vinylphosphonic acid groups.
 7. The process as claimed inclaim 6, characterized in that the acid is phosphoric acid and themonomer containing vinylsulfonic acid groups is vinylsulfonic acid andthe monomer containing vinylphosphonic acid groups is vinylphosophonicacid.
 8. The process as claimed in claim 1, characterized in that thefilm is passed through the bath of liquid at least 10 times.
 9. Theprocess as claimed in claim 1, characterized in that the film is passedthrough the bath of liquid at a speed of from 0.5 to 100 m/min.
 10. Theprocess as claimed in claim 1, characterized in that the film isconveyed with a drawback force per unit width of the film in the rangefrom 0.5 to 200 N/m.
 11. The process as claimed in claim 1,characterized in that the film is treated for from 15 minutes to 3hours.
 12. The process as claimed in claim 1, characterized in that ajigger is used for the treatment of polyazole films.
 13. The process asclaimed in claim 2, characterized in that liquid adheres to the filmafter the first treatment.
 14. The process as claimed in claim 13,characterized in that at least 10 g/m² of liquid adheres to the film.15. The process as claimed in claim 14, characterized in that the liquidcomprises water and from 25 to 75 ml/m² of water adhere to film.
 16. Theprocess as claimed in claim 15, characterized in that the liquidcomprises acid, monomers containing vinylsulfonic acid and/or monomerscontaining vinylphosphonic acid groups.
 17. The process as claimed inclaim 16, characterized in that the acid is phosphoric acid and themonomer containing vinylsulfonic acid groups is vinylsulfonic acid andthe monomer containing vinylphosphonic acid groups is vinylphosophonicacid.
 18. The process as claimed in claim 17, characterized in that thefilm is passed through the bath of liquid at least 25 times at a speedof from 1.0 to 25 m/min.
 19. The process as claimed in claim 18,characterized in that the film is conveyed with a drawback force perunit width of the film in the range from 12 to 60 N/m and the film istreated for from 15 minutes to 3 hours.
 20. The process as claimed inclaim 19, characterized in that a jigger is used for the treatment ofpolyazole films.